PROJECT SUMMARY/ABSTRACT Extreme heat is a significant public health threat that is expected to worsen due to climate change. To reduce heat-related morbidity and mortality, the United States (US) National Weather Service (NWS) typically issues heat warnings to communicate risks to the public when the heat index (HI) for the next day is forecast to be above a threshold value. However, few studies have examined whether heat warnings prevent deaths, and no studies have examined whether heat warnings prevent morbidity. Accordingly, we propose to quantify the public health benefit of heat warnings issued by NWS in the large, national population of Medicare beneficiaries. As heat warnings are issued based on forecast HI, and forecasts are sometimes wrong, in any given location there exists a set of hot days of similar HI with and without heat warnings. We will take advantage of this data structure by using case-crossover analyses to compare days of similar HI with and without heat warnings. Specifically, we will address the following Specific Aims using all deaths and all emergency hospitalizations among Medicare beneficiaries between 2001 and 2012 in 207 US cities. In Aim 1, we will assess whether heat warnings are associated with reduced risk of death among US Medicare beneficiaries. In Aim 2, we will assess whether heat warnings are associated with reduced morbidity as assessed by risk of emergency hospitalization among US Medicare beneficiaries. In Aim 3, we will assess how the benefits of heat warnings vary across levels of individual, community, and meteorologic characteristics. To accomplish these Aims, this proposal will leverage a valuable but previously unutilized data source: a national database of text records of heat warnings issued by the NWS. This proposed research is significant because the findings will provide public health and emergency preparedness officials across the country with critical information they need to better protect the public's health during extreme heat events. For example, the results have the potential to inform NWS policy regarding the criteria for issuing warnings and the frequency with which warnings are issued. The results could further inform actions taken by local health and emergency management offices to mitigate the impact of extreme heat events, such as the development of heat preparedness and response plans. This proposed research is well- aligned with NIEHS's strategic goal of identifying and responding to emerging environmental threats to human health.